This invention relates to a method for pressurizing containers by means of dispensing liquefied nitrogen into metal cans made of relatively thin aluminum or steel material, sealing the cans and then causing the liquefied nitrogen to gassify in the cans so as to preserve the contents of the cans and to create pressure therein to impart to them sufficient strength. The invention is suitable for preparing cans filled with liquid beverages or drinks not containing carbon dioxide.
A drawing and ironing method is now being widely used to manufacture what are referred to as two-piece cans made of steel or aluminum sheets having a wall thickness of about 0.15 mm. These two-piece cans as they are called are used for containing beverages because they are strong, efficient, lightweight and can be quickly heated or cooled. Aluminum cans also have the advantage that most liquids contained therein will not deteriorate or degrade in taste even though the inner surface of the cans is not subjected to any special treatment. In addition, since aluminum has an excellent workability, it is possible to integrally form its body or cylindrical portion and bottom portion, thereby reducing the manufacturing cost.
When the thin walled aluminum or steel cans are charged with such liquid beverages as beer or beverage containing carbon dioxide, the carbon dioxide liberated creates an internal pressure sufficient to increase the mechanical strength of the cans. However, when the cans are filled with liquid beverages not containing carbon dioxide, for example, coffee, fruit juice, plain water and wine, the internal pressure is not created. Accordingly, for use in such liquid beverages not containing carbon dioxide, one alternative has been to increase the wall thickness for the purpose of increasing the mechanical strength of the cans, and thin sheets manufactured by the drawing and ironing method cannot be used.
The general concept of adding liquid nitrogen to thin-walled containers has been broadly disclosed in U.K. Pat. No. 1,455,652; however, in that instance, the details of the apparatus are very sketchy. German OLS No. 3,141,465 published July 15, 1982, includes additional apparatus; however, this is concerned with specific details unrelated to the method herein. Japanese published applications Nos. 56-4521, 49-4389 and 145686/76, U.S. Pat. No. 2,978,336 and Canadian Pat. No. 1,062,671 also disclose use of liquid nitrogen in packaging but are not considered relevant to the present invention.